The present invention relates to magnetic elements with stable but switchable domain structures. These elements are useful in array Magnetic Random Access Memories (MRAM) and in hard disk read/write heads.
The use of magnetic structures in magnetic data storage has intensified during the last 10 years. The main application is found in the read head in hard disk drives, which are based on Anisotropic Magneto Resistive (AMR) thin films and on more complex systems utilising Giant Magneto Resistive (GMR), or Tunnel Magneto Resistive (TMR), multilayers. Another newer application is the use of small magnetic elements to store the data in Random Access Memories. The main qualities needed for these applications, apart from a high magneto resistance, are well defined and reproducible magnetic states having a predictable response to an applied magnetic field. In addition, with the increase in speed in microelectronics, the required switching speed between different magnetic states is approaching the GHz range.
The choice of geometry and the equilibrium state of the magnetic element are crucial in achieving this performance. All existing devices in use today are topologically simply connected i.e. disc shaped, square, rectangular or even eye shaped. One problem with these structures is edge effects, defining a multitude of different and complex states with similar energy. This leads to complex switching behaviour. The multitude of states in these structures makes them very sensitive to imperfections, both macroscopic defects, i.e. shape variation, and atomic scale defects resulting from the pattering. It has been shown by Zhu et al. “Journal of Applied Physics,” page 6668-6673, Volume 87, Number 9., that a slight variation in the shape of a rectangular element will result in 100% change in the coercive field.
To obtain a better defined state, Zhu et al. proposed to use a connected element in the shape of a ring, for which the vortex state is very stable. Information can be stored as the two opposite vortex states. The switching of these elements is, however, complicated and the magnetic elements pass through a complex intermediate state with a radial magnetisation. This complexity sets a limit on the size of usable structures and the switching speed.
There is therefore a need for non-volatile memory arrays and magnetic read/write heads for devices which have greater speed, reliability and cost effectiveness.